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Discussion Starter #1
There is a screetching sound coming from underneath the car when I drive, but it only happens when I push the gas pedal to accelerate. The screeching sound isnt there if Im stopped and idling or when Im coasting with the foot of the gas pedal. This makes me thing that its a transmission slipping problem and not a belt problem, especially since the car seems slower to accelerate than it should.

Now, this only happens for the first couple of minutes right after I start the car. After that, the screeching stops and the transmission shifts smoother and the car seems to accelerate faster. Now, I can drive the car for hours on end without any more screeching after it initially stops, and then shut down the car and start it up again a couple minutes later with the engine still hot but the screeching begins again. This makes me think that the problem is related to the vacuum system somehow.

Am I correct in my assumption? If so, what is the specific culprit or how do I start hunting down the root cause? Are there any specific vacuum-transmission parts that are prone to failure?

Any advice would be greatly appreciated.
 

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I can't really say that I have encountered this problem myself, but I would say to use a mityvac and test the modulator on the transmission. Hopefully the screeching you hear is just a vacuum leak. If that is okay, reference the vacuum diagrams and try working your way from the transmission to the vacuum pump testing all the other connections. Also check you fluids. If the tranny oil smells burnt and is very dark, you may have a serious problem.
 

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first vacuum test

I tested the system while the engine was off, I hooked the mityvac to the line that goes to the transmission and it held all the vacuum I put on it. Then I plugged the line back into the 3-way tee and hooked the pump up to the tee end where the restricted orifice connects the two 3-way tees. The vacuum would not go beyond 10" of vacuum but would hold it stable-which appears to be correct since the dampener and proportioned vacuum leak created by the diesel injection vacuum control valve keeps the vacuum at 10-15" as a max and decreases it to 0" at full throttle.

Tomorrow I will test the total vacuum at first branch after the vacuum pump, Im thinking there might be soot in the lines or something that is restricting vacuum throughout the entire system. I will also start replacing vacuum lines, 3-way tees, and restricted orafices before I replace the vacuum 3/2 way vacuum control valve or the transmission modulator valve.
 
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